1. Field of the Invention
Embodiments of the present invention generally relate to systems and method for producing ammonia and ammonia products. More particularly, embodiments of the present invention relate to systems and methods using high pressure cryogenic separation for producing ammonia and ammonia products.
2. Description of the Related Art
Synthesis gas or simply “syngas” is typically the product of gasifying a carbon containing material in the presence of an oxidant at high temperatures and pressures. Depending on the starting materials, syngas is usually rich in carbon monoxide and hydrogen, and can also contain varying amounts of methane, carbon monoxide, nitrogen, and argon.
As the name suggests, syngas is often used for the synthesis of chemicals or synthetic hydrocarbon fuels. Syngas can be used as a fuel to generate electricity or steam and as a source of hydrogen. Syngas can also be used to produce a wide range of products, such as fuels, chemicals, fertilizers, and industrial gases.
Ammonia, for example, is commonly produced from syngas. An ideal syngas for ammonia (NH3) synthesis will have a molar ratio of 3 moles hydrogen to 1 mole of nitrogen. Contaminants such as oxygen, methane, and excess nitrogen can detrimentally interfere with the ammonia conversion reaction and are removed prior to introducing the syngas to the ammonia synthesis reactors. Various methods and technologies for separating those undesirable gaseous components to yield a purified syngas containing a pure syngas having a 3:1 molar ratio of hydrogen to nitrogen have been used. For example, membranes, molecular sieves, and/or cryogenic separation techniques have been employed, alone or in combination, to remove such contaminants to yield a synthesis gas having the appropriate molar ratio.
Cryogenic separation is a frequently employed technique for separating contaminants and excess nitrogen from the raw syngas. Using this technique, the raw syngas is typically fed through a cryogenic separation system prior to compressing the syngas for feed to a synthesis reactor. Within the cryogenic separation system, contaminants, including excess nitrogen, carbon monoxide, methane and argon, are scrubbed from the syngas using a cryogenic fluid such as liquid nitrogen. The contaminants are removed and the purified syngas, containing the optimal ratio of hydrogen to nitrogen is recovered, compressed and introduced to the ammonia synthesis reactor where the nitrogen and hydrogen react, forming ammonia.
However, the syngas pressure entering the cryogenic separation system is frequently insufficient to provide the necessary refrigeration required to separate the contaminants; thus, additional compression of the syngas is required upstream of the cryogenic separation system. The pressure drop through the cryogenic separation system (approximately 300 kPa (30 psig) to 500 kPa (60 psig)) can also decrease the available pressure on the suction side of the ammonia synthesis compressor; thereby requiring additional power to compress the syngas feed to the ammonia synthesis reactor pressure requirements.
There is a need, therefore, for an improved, more energy efficient process for removing contaminants, including excess nitrogen, carbon monoxide, methane and argon from a syngas prior to ammonia production.